Devices in which a piezoelectric phenomenon is utilized are used in various fields. The devices have been increasingly used in portable devices (e.g. a cellular phone) which have been strongly required to be further miniaturized and smaller in power consumption. As examples of the devices, an IF (intermediate frequency) filter, an RF (radio frequency) filter, and the like are given. Specific examples of the IF filter and the RF filter encompass a SAW filter which is a filter including a surface acoustic wave resonator (SAWR), and the like.
The SAW filter is a filter including a resonator in which an acoustic wave transmitted on a solid surface is utilized. Design and a manufacturing technique of the SAW film have been improved to thereby meet severe demands of users. However, the improvement of the SAW filter is approaching to its limit, along with an increase in frequency to use.
In view of the circumstances, an FBAR filter including a film bulk acoustic resonator (FBAR) has been developed as a new filter in place of the SAW filter. The FBAR filter is one of RF-MEMS (radio frequency-micro electro mechanical system) devices.
The RF-MEMS is a technique which has been drawn the attention in recent years. The RF-MEMS is formed by applying an MEMS to an RF front end. The MEMS is a technique for forming a device (e.g., a very small actuator, sensor, resonator, etc.) by forming a mechanical microstructure mainly on a semiconductor substrate.
The FBAR filter, which is one of the RF-MEMS devices, is a filter including the resonator in which a thickness longitudinal vibration mode of a thin-film for indicating piezoelectric response is utilized. Specifically, the FBAR filter includes the resonator in which the following phenomenon is utilized: a piezoelectric thin film receives a high-frequency electric signal to thereby generate thickness longitudinal vibration and then the vibration generates resonance in a thickness direction of the thin film. Such FBAR filter can resonate in a gigahertz band. The FBAR filter having such property has low loss and can be operated in a wide band, and in addition, makes it possible to attain further miniaturization and electric power saving of portable devices.
Further, the RF-MEMS devices (in addition to the FBAR filter, e.g., an RF-MEMS capacitor and an RF-MEMS switch) in which the piezoelectric phenomenon is utilized attain low loss, high isolation, and low strain in a high-frequency band.
Patent Literature 1 discloses that a piezoelectric thin film including an aluminum nitride thin film which is added with scandium as a third component has satisfactory piezoelectric response.